Processing and transmission of information with light requires creation of integrated optical circuits. While the idea is not novel, integrated circuits with the use of light do not repeat the success of electronic integrated circuits, while most important active and non-linear optic elements like lasers, amplifiers, detectors, and fast saturating absorbers, are routinely made in planar waveguides with microlithography, then diced and connected with optical fibers. It is much like the use of transistors before the invention of electronic integrated circuits. One of the main reasons is the problem of interconnection. Electric current easily follows through bends of a conductor, thereby facilitating interconnections among several layers. The light tends to propagate in a straight line; therefore, interconnections among several layers are difficult. Sometimes active elements are interconnected by ridge waveguides in a single waveguide, but this method is limited due to the crossing of ridge waveguides in a single layer. Thus, there is a great need for interconnecting many optical elements in a single waveguide.
Attempts have been made heretofore to provide planar optical devices by interconnecting many optical devices on a single substrate. For example, U.S. Patent Application Publication No. 20070034730 published in 2007 (inventor T. Mossberg, et al.) discloses a method that comprises the steps of forming a planar optical waveguide that confines in one transverse spatial dimension an optical signal propagating in two other spatial dimensions, and forming a set of diffractive elements in the planar optical waveguide. The latter is arranged so as to support multiple optical transverse modes in the confined transverse dimension. Each diffractive element set is arranged so as to route, between a corresponding input optical port and a corresponding output optical port, a corresponding diffracted portion of the optical signal propagating in the planar waveguide that is diffracted by the diffractive element set. The diffractive elements are arranged so that the optical signal is successively incident on the diffractive elements; and the diffractive elements and the planar optical waveguide are arranged so that the corresponding diffracted portion of the optical signal reaches the corresponding output optical port as a superposition of multiple optical transverse modes supported by the planar optical waveguide.
U.S. Patent Application Publication No. 20060233493 published in 2006 (inventor T. Mossberg, et al.) discloses a method comprising the steps of receiving an input optical signal successively incident on a set of diffractive elements in an optical medium. The optical medium enables substantially unconfined propagation of optical signal in three dimensions. At least a portion of the input optical signal passes through the set of diffractive elements and produces an output optical signal. The diffractive elements of the set are collectively arranged within the slab waveguide so as to exhibit a positional variation in amplitude, optical separation, or spatial phase over some portion of the set. Furthermore, the diffractive elements of the set collectively apply a transfer function to the input optical signal for producing the output optical signal, the transfer function being determined at least in part by said positional variation in amplitude, optical separation, or spatial phase exhibited by the diffractive elements of the set.
U.S. Patent Application Publication No. 20070053635 published in 2007 (inventor D. Lazikov, et al) discloses a method that comprises computing an interference pattern between a simulated design input optical signal and a simulated design output optical signal, and computationally deriving an arrangement of at least one diffractive element set from the computed interference pattern. The interference pattern is computed in a transmission grating region, with the input and output optical signals each propagating through the transmission grating region as substantially unconfined optical beams. The arrangement of diffractive element set is computationally derived so that when the diffractive element set thus arranged is formed in or on a transmission grating, each diffractive element set would route, between corresponding input and output optical ports, a corresponding diffracted portion of an input optical signal incident on and transmitted by the transmission grating. The method can further comprise forming the set of diffractive elements in or on the transmission grating according to the derived arrangement.
U.S. Patent Application Publication No. 20060126992 published in 2006 (inventor T. Hashimoto, et al.) discloses a wave transmission medium that includes an input port and an output port. The first and the second field distributions are obtained by numerical calculations. The first field distribution distributes the forward propagation light launched into the input port. The second field distribution distributes the reverse propagation light resulting from reversely transmitting from the output port side an output field that is sent from the output port when an optical signal is launched into the input port. A spatial refractive index distribution is calculated on the basis of both field distributions such that the phase difference between the propagation light and reverse propagation light is eliminated at individual points (x, z) in the medium. The elements of this system are also mounted on a planar substrate.
U.S. Patent Application Publication No. 20040036933 published in 2004 (inventor V. Yankov, et al.) discloses a method and device that provide efficient wavelength division multiplexing/demultiplexing (WDM) including reduced signal distortion, higher wavelength selectivity, increased light efficiency, reduced cross-talk, and easier integration with other planar devices, and lower cost manufacturing. The method and device include a planar holographic multiplexer/demultiplexer having a planar waveguide, the planar waveguide including a holographic element that separates and combines pre-determined (pre-selected) light wavelengths. The holographic element includes a plurality of holograms that reflect pre-determined light wavelengths from an incoming optical beam to a plurality of different focal points, each pre-determined wavelength representing the center wavelength of a distinct channel. Advantageously, a plurality of superposed holograms may be formed by a plurality of structures, each hologram reflecting a distinct center wavelength to represent a distinct channel to provide discrete dispersion. When used as a demultiplexer, the holographic element spatially separates light of different wavelengths and when reversing the direction of light propagation, the holographic element may be used as a multiplexer to focus several optical beams having different wavelengths into a single beam containing all of the different wavelengths.
However, in all aforementioned prior-art devices, for transformation of an input beam into an output beam, the inventors use holographic gratings with known functional properties determined by their parameters and geometry. Therefore, positions and optical parameters of the input and output beams strictly depend on the geometry of the grating, and this significantly limits design of the optical structure. Another disadvantage of the known methods and planar holographic devices is that they can provide a limited number of light-transmitting channels since each holographic pattern element works only with one or two channels.